1. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to the transmission of control signals conveying scheduling assignments for data reception or data transmission in multiple distinct bandwidths of a communication system.
2. Description of the Art
Unicast communication systems consist of a DownLink (DL) and of an UpLink (UL). The DL conveys transmissions of signals from a serving Base Station (BS or Node B) to User Equipments (UEs). The DL signals consist of data signals carrying the information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The data signals are transmitted from the serving Node B to the respective UEs through the Physical Downlink Shared CHannel (PDSCH). The UL conveys transmissions of signals from UEs to their serving Node B. The UL signals also consist of data signals, control signals, and RSs. The data signals are transmitted from UEs to their serving Node B through the Physical Uplink Shared CHannel (PUSCH).
A UE, which is also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other related terminology.
The DL control signals may be of broadcast or UE-specific (unicast) nature. Broadcast control signals convey system information to all UEs. Further, UE-specific control signals can be used, among other purposes, to provide to UEs Scheduling Assignments (SAs) for PDSCH reception (DL SAs) of PUSCH transmission (UL SAs). The transmission of UE-specific control signals from the serving Node B to UEs is commonly through the Physical Downlink Control CHannel (PDCCH). The UL control signals include acknowledgement signals associated with the application of Hybrid Automatic Repeat reQuest (HARQ) for PDSCH transmissions and Channel Quality Indication (CQI) signals informing the serving Node B of the channel conditions the UE experiences in the DL. In the absence of any data transmission, a UE transmits these control signals through the Physical Uplink Control CHannel (PUCCH).
Typically, the PDCCH is a major part of the total DL overhead and directly impacts the achievable DL throughput. One method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the SAs during each PDSCH Transmission Time Interval (TTI). In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), where the Node B uses Orthogonal Frequency Division Multiple Access (OFDMA) as the DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through the Physical Control Format Indicator CHannel (PCFICH) indicates the number of OFDM symbols occupied by the PDCCH.
FIG. 1 illustrates the PDCCH transmission in the DL TTI, which for simplicity, is assumed to consist of one sub-frame having M OFDM symbols.
Referring to FIG. 1, the PDCCH 120 occupies the first N symbols of the total symbols 110. The remaining M-N symbols 130 of the sub-frame are assumed to be primarily used for the PDSCH transmission. The PCFICH 140 is transmitted in some sub-carriers, which are also referred to as Resource Elements (REs), of the first symbol. Certain sub-frame symbols also contain RS REs 150 and 160 for each of the Node B transmitter antennas, respectively, which in FIG. 1 are assumed to be two. The main purposes of the RS are to enable a UE to obtain a channel estimate for its DL channel medium and to perform other measurements and functions.
Alternatively, additional control channels may also be transmitted in the PDCCH region 120, even though they are not illustrated in FIG. 1. For example, assuming the use of HARQ for PUSCH data transmissions, a Physical Hybrid-HARQ Indicator CHannel (PHICH) may be transmitted by the Node B in a similar manner as the PCFICH to indicate to groups of UEs whether or not their previous PUSCH transmission was correctly received by the Node B.
The Node B may separately code and transmit each of the DL SAs and UL SAs in the PDCCH.
FIG. 2 illustrates a processing chain for an SA coding.
Referring to FIG. 2, the SA information bits 210, which convey the information for PDSCH reception or PUSCH transmission to a UE, are appended Cyclic Redundancy Check (CRC) bits in step 220, and are subsequently encoded in step 230, for example using a convolutional code, rate matched to the assigned resources in step 240, and transmitted in step 250. Consequently, each UE performs multiple decoding operations in its respective PDCCH region to determine whether it is assigned a DL SA or an UL SA. Typically, the CRC of each SA is scrambled with the IDentity (ID) of the UE the SA is intended for (not shown). After descrambling with its ID, a UE can determine whether an SA is intended for it by performing a CRC check.
In FIG. 3, the inverse operations of those illustrated in FIG. 2 are performed for SA decoding at the UE receiver.
Referring to FIG. 3, the received SA 310, is rate de-matched in step 320, decoded in step 330, and then the CRC is extracted in step 340. After CRC extraction, the SA information bits are obtained in step 350. As described above, if the CRC check passes, the UE may consider the SA as its own.
The SA information bits correspond to several fields such as, for example, a Resource Allocation (RA) field indicating the part of the operating BandWidth (BW) allocated to a UE for PDSCH reception (DL SA) or PUSCH transmission (UL SA), a Modulation and Coding Scheme (MCS) field, a field related to the HARQ operation, etc. Normally, the BW unit for PDSCH or PUSCH transmissions consists of several REs, such as, for example, 12 REs, and will be referred to herein as a Resource Block (RB).
In order to assist a UE with the multiple decoding operations, the REs carrying each SA are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of SA bits in FIG. 2, the number of CCEs for the SA transmission depends on the channel coding rate (e.g., Quadrature Phase Shift Keying (QPSK) as the modulation scheme). For a UE with low Signal-to-Interference and Noise Ratio (SINR), the serving Node B may use a low channel coding rate for the respective SA transmission in order to achieve a desired BLock Error Rate (BLER). For a UE with high SINR, the serving Node B may use a high channel coding rate for the respective SA transmission in order to achieve the same desired BLER. Therefore, the SA transmission to a UE experiencing a high SINR in the DL of the communication system typically requires more CCEs than that the SA transmission to a UE experiencing a low SINR (different power boosting of the REs used for a CCE transmission may compensate to an extent for the difference in coding rates in order to achieve the same SA BLER). Typical CCE aggregations for an SA transmission are assumed to follow a “tree-based” structure consisting, for example, of 1, 2, 4, and 8 CCEs.
For the SA decoding process, a UE may determine a search space for candidate SAs, after it restores the CCEs in the logical domain (prior to CCE interleaving), according to a common set of CCEs for all UEs (UE-common search space) and a UE-specific set of CCEs (UE-specific search space). The UE-specific search space may be determined according to a pseudo-random function having as inputs UE-common parameters, such as the sub-frame number or the total number of CCEs, and UE-specific parameters such as the identity assigned to a UE (UE_ID).
For example, in 3GPP LTE, for CCE aggregation levels L∈{1, 2, 4, 8}, the CCEs corresponding to SA candidate m are given by L·{(Yk+m)mod └NCCE,k/L┘}+i where NCCE,k is the total number of CCEs in sub-frame k, i=0, . . . , L−1, m=0, . . . , M(L)−1, and M(L) is the number of SA candidates to monitor in a search space (UE-common or UE-specific). Exemplary values of M(L) for L∈{1, 2, 4, 8} in the UE-specific search space are, respectively, {6, 6, 2, 2}. The variable Yk is defined as Yk=(A·Yk-1)mod D, where Y−1=UE_ID≠0, A=39827 and D=65537.
FIG. 4 illustrates construction and transmission of SAs using CCEs.
Referring to FIG. 4, the CCEs are serially numbered in the logical domain 400. After channel coding and rate matching, as shown in FIG. 2, the encoded SA bits are mapped to CCEs in the logical domain. More specifically, the first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used for the SA transmission to UE1. The next 2 CCEs (L=2), CCE5 411 and CCE6 412, are used for the SA transmission to UE2. The next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used for the SA transmission to UE3. Finally, the last CCE (L=1), CCE9 431, is used for the SA transmission to UE4.
The SA bits may be scrambled in step 440 using binary scrambling code, which is typically cell-specific, and are subsequently modulated in step 450. Each CCE is further divided into mini-CCEs. For example, a CCE consisting of 36 REs can be divided into 9 mini-CCEs, each consisting of 4 REs. Interleaving is applied among mini-CCEs (blocks of 4 QPSK symbols) in step 460. For example, a block interleaver, as used in 3GPP LTE, may be used where the interleaving is performed on symbol-quadruplets (4 QPSK symbols corresponding to the 4 REs of a mini-CCE) instead of on individual bits.
After interleaving the mini-CCEs, the resulting series of QPSK symbols may be shifted by J symbols in step 470, and then each QPSK symbol is mapped to an RE in the PDCCH region of the DL sub-frame in step 480.
Accordingly, in addition to the RS from the two Node B transmitter antennas 491 and 492 and other control channels, such as the PCFICH 493 and the PHICH (not shown), the REs in the PDCCH contain QPSK symbols corresponding to the SAs for UE1 494, UE2 495, UE3 496, and UE4 497.
In order to support higher data rates and enable scheduling of signal transmissions over BWs larger than the BWs of Component Carriers (CCs) supporting legacy communication systems, aggregation of multiple CCs is typically considered. For example, to support communication over 100 MHz, aggregation of five 20 MHz CCs can be used. For ease of reference herein, UEs operating over a single CC according to a pre-existing communication method will be referred to as “legacy-UEs” and UEs operating over multiple CCs will be referred to as “advanced-UEs”.
Enabling the coexistence of SAs for legacy-UEs and advanced-UEs and designing the transmission of SAs for advanced-UEs are among the fundamental issues to be solved for the support of communications over multiple CCs.
FIG. 5 illustrates the principle of CC aggregation.
Referring to FIG. 5, an operating BW of 100 MHz 510 is constructed by the aggregation of 5 (contiguous, only for simplicity) CCs 521, 522, 523, 524, and 525, each having a BW of 20 MHz. Similarly to the sub-frame structure for communication over a single CC in FIG. 1, the sub-frame structure for communication over multiple CCs consists of a PDCCH region, such as for example 531 through 535, and a PDSCH region, such as for example 541 and 545.
The PDCCH region size varies per CC and its value is signaled by the PCFICH in the respective CC for the reference sub-frame period. By allowing the PDCCH to have a variable size, the respective overhead is minimized while practically avoiding PDSCH or PUSCH scheduling restrictions. Additionally, by configuring an advanced-UE to receive its PDSCH in predetermined CCs, the advanced-UE will only decode the PCFICH in these CCs and not all CCs, thereby minimizing the impact of PCFICH decoding errors. For CCs 1 and 5, the PDCCH size is respectively, PDCCH-1=3 symbols 531 and PDCCH-5=1 symbol 535. Because the PDSCH size in each CC is found by subtracting the respective PDCCH size from the sub-frame size, it is PDSCH-1=11 symbols 541 and PDSCH-5=13 symbols 545.
FIG. 5 also illustrates the direct extension of the PDCCH design for SA transmissions to advanced-UEs. The scheduling is independent among CCs and is performed by a PDCCH that is included within its respective CC, regardless of the number of CCs an advanced-UE may use for its PDSCH reception or PUSCH transmission. For example, the advanced-UE 550 receives two distinct SAs, SA2 552 and SA3 553, for individual PDSCH reception in the second and/or third CCs, respectively, and the advanced-UE 560 receives SA5 565 for PDSCH reception in the fifth CC. Different transport blocks are associated with different SAs.
However, a disadvantage of using an individual SA in each CC is that the advanced-UE will perform as many as 5 times (for the exemplary setup of 5 CCs in FIG. 5) the number of decoding operations a legacy-UE has to perform in order to identify the SAs in all possible CCs.
Another design issue is the multiplexing of CCEs corresponding to SAs for legacy-UEs and advanced-UEs without affecting the SA decoding process for legacy-UEs or increasing the number of decoding operations for advanced-UEs.